So-called three-way catalysts have the characteristics of converting the harmful substances CO, HC and NO.sub.x in a determined range of the composition of the exhaust gas. At the present time, they are being tested in motor vehicles in connection with controlled carburetor or an injection installation ("closed loop") for the purification of exhaust gas. By means of an oxygen-measuring probe, the so-called .lambda.-probe, the O.sub.2 content in the exhaust gas is determined. The probe produces a voltage between 200 and 800 mV, depending on the O.sub.2 partial pressure in the exhaust gas. The voltage is amplified electronically and by way of a controller, it controls the formation of the mixture of the injection system or of a carburetor. The control is accomplished such that the composition of the exhaust gas mixture always makes possible the optimal simultaneous conversion of CO, HC and NO.sub.x at the most favorable operating point of the catalyst in the narrowest possible band width. The following assumptions are necessary for an effective operation of the controlling system and the catalyst:
1. The composition of the mixture should be as stoichiometric as possible, i.e., in the case of a complete conversion of the exhaust gas components, only the reaction products CO.sub.2, H.sub.2 O and N.sub.2 would remain (.lambda.=1);
2. The optimal operating point of the catalyst should lie as close as possible to .lambda.=1 or beyond that in the lean exhaust gas range;
3. The characteristic line of the .lambda.-probe should coincide as much as possible with the optimal operating point of the catalyst at the most stable control point.
In the case of known three-way catalysts and the .lambda.-probes being used serially at the present, the optimal operating and control points clearly deviate from each other. Thus, for example, the range in which the highest CO and NO.sub.x conversion is achieved in dynamic operation lies between .lambda.=0.985 0.990, corresponding to the air/fuel ratio A/F=14.48-14.55, while the .lambda.-probe has its optimal operating point at 350 mV, corresponding to .lambda.=1.001.
Thus, there is a difference between the optimal operating points of the catalyst and the .lambda.-probe of the .DELTA..lambda.=0.016-0.011, a disadvantage which must be compensated by the control. With an electronic control, this will be possible only to a limited degree by way of the regulating time constant, which leads to the fact that the catalyst is acted upon with an exhaust gas which has a different composition as would be required for the maximum conversion of all harmful substances. Continuing progressive legislation in the United States which requires a reduction of the NO.sub.x emission beginning with 1982 to 0.42 g/M therefore requires a catalyst, the optimal operating point of which lies closer to .lambda.=1.